Numerous methods for controlling the power supply from a power source to a power consumer are known from the related art, where a certain input value is preselected by a control unit. With the digital control units customary today, a binary number is used as the input value, e.g., an 8-bit number which may represent an integer between 0 and 255.
In the method known as pulse width modulation, as described in the white paper of Artistic Licence (UK) Ltd., Great Britain, dated May, 2002, entitled “An overview of the electronic drive techniques for intensity control and colour mixing of low voltage light sources such as LEDs and LEPs”, for example, the operating time of the power consumer is divided into successive periods. Each period is first switched on and switched off at a point in time within this period which depends on the setpoint value to be set.
The use of this method for controlling the power supply to light sources, in particular LEDs, via a microcontroller is known from EP 1 016 062 B1, for example.
Pulse width modulation is performed with the help of a counter which is incremented in integral increments up to a maximal value during each period, the control signal being initially set high at the start of each period, i.e., at the lowest count value. This means that the period starts with an ON segment. As soon as the count value corresponds to the brightness setpoint value stored in the register, the control signal is set low, so that the OFF segment of the period is initiated. In this method, a check is performed at each count value of the counter during a period, i.e., typically 255 times in the case of an 8-bit control, to ascertain whether it is necessary to switch from the high control signal to the low control signal. This is very complicated and is associated with the use of a large amount of computation resources for the particular program control. When using the method for illumination control, very short period lengths in the range of a few milliseconds are required to prevent a visible flickering effect. With such short period lengths, most of the computation time available during a period is occupied by checking on whether the system is to be shut down at the next count value. Under some circumstances adequate computation time is no longer available on the microcontroller for implementing other tasks.
An alternative method known as bit angle modulation is also described in the white paper of Artistic Licence (UK) Ltd., Great Britain, referenced above.
With this known method, the period length is divided into fixed blocks of different lengths in the manner of the binary system so that starting with the block of the shortest length, the next-longer block is doubled in length in comparison with the next-shorter block. At the beginning of each block, the control state may be altered. Within the blocks, the control state cannot be altered. Each block may be assigned to a bit within the binary input value. The longest block is assigned to the most significant bit and the shortest block is assigned to the least significant bit. When a bit is occupied with 1 the assigned block is switched on. Due to this combination of switched on and/or switched off blocks, preselected binary input values are directly converted to ON times. Within each block the control state does not change so that no computation time is engaged by the control during the length of each block. The computation time of the microcontroller which is thus free in comparison with pulse width modulation is now available for implementation of other tasks.
In both pulse width modulation and bit angle modulation, the ON period within the period is controllable as a proportional function of a binary input value. In other words, a change in the input value takes place in integral increments, one increment being assigned a fixed segment of time. In the case of an 8-bit input value which may represent integral values between 0 and 255, for example, exactly the same number of ON periods are implemented, the time segment of the ON period for each integral increment of the input value constituting 1/255 or 0.392% of the entire period.
However, proportional mapping of the input value often does not correspond to the ratio between the power supplied and the perceived increase in intensity. In the case of light sources, for example, the perceived increase in intensity is a logarithmic function of the power supplied. This means that at a low luminous output, even a very minor increase in power supply results in a substantial increase in intensity. At a high luminous output, however, increases in power supply result in less perceived intensity. The same thing also applies to sound sources. When a sound source is operated quietly, a minor increase in power supply results in a great increase in loudness. In a loud environment, however, an increase in power supply results only in a minor increase in loudness.
However, the input value often reflects the subjective output, e.g., the subjectively perceived brightness or loudness. Therefore, tables for mapping the input values via an exponential function are frequently used, so that the power supply increases exponentially and an essentially linear relationship between the input value and the subjectively perceived power (e.g., brightness) is achieved. Mapping via an exponential function in the range of low powers results in very minor changes in power per increment of the input value. The changes are frequently much smaller than the smallest time segment by which the ON period may be varied. In this case, the same ON periods are assigned to different successive input values. On the other hand, at a high power, the resolution of the known methods is too high. A plurality of possible regulating steps is skipped directly with one increment of the original input value by using the table which maps the input values via an exponential function, so that multiple time segments of the ON period are added for each increment at the high power values.
Accordingly, it would desirable to create a method for controlling the power supply from a power source to a power consumer which will permit a high resolution with a low processor load.